Allow case-insensitive scheme as input

[dwalluck]

The spec should allow a case-insensitive scheme as input as per https://datatracker.ietf.org/doc/html/rfc3986#section-6.2.2.1 and https://en.wikipedia.org/wiki/URI_normalization. Note that this suggestion doesn't change anything else about the lowercase "pkg" being the normalized form.

The scheme (and host) components of the URI are case-insensitive and therefore should be normalized to lowercase. Example: HTTP://[email protected]/Foo → http://[email protected]/Foo